The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle.
Due to increased consumer demand for four-wheel drive vehicles, a plethora of different power transfer systems are currently utilized for directing power (i.e., drive torque) to all four wheels of the vehicle. For example, in many xe2x80x9cpart-timexe2x80x9d power transfer systems, a transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. When four-wheel drive is desired, a xe2x80x9cmodexe2x80x9d shift mechanism can be selectively actuated by the vehicle operator for directly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. As will be appreciated, motor vehicles equipped with such a part-time power transfer systems offer the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions. An example of a part-time transfer case is disclosed in commonly-owned U.S. Pat. No. 4,770,280 to Frost.
Alternatively, it is known to use xe2x80x9con-demandxe2x80x9d power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, the xe2x80x9con-demandxe2x80x9d feature is incorporated into the transfer case by replacing the mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque xe2x80x9con-demandxe2x80x9d to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an xe2x80x9con-demandxe2x80x9d power transfer system is disclosed in commonly-owned U.S. Pat. No. 5,323,871 to Wilson et al wherein the electronically-controlled clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference (i.e., the wheel slip) between the driven and non-driven wheels.
As a further alternative, some vehicles are equipped with full-time power transfer systems having a transfer case equipped with a center differential that functions to permit interaxle speed differentiation while transferring drive torque to both of the front and rear drivelines. To minimize loss of traction due to wheel slippage, many full-time transfer cases are also equipped with a clutch assembly for limiting speed differentiation and biasing the torque transferred across the center differential. For example, full-time transfer cases are disclosed in commonly-owned U.S. Pat. No. 5,697,861 to Wilson and U.S. Pat. No. 5,702,321 to Bakowski et al. Whit are respectively equipped with mechanically-actuated and hydraulically-actuated clutch assemblies for controllably modulating the drive torque delivered across the center differential to the front and rear drivelines.
While the power transfer systems disclosed above have advanced the technology associated with four-wheel drive motor vehicles, a need still exists to develop alternative arrangements which continue to provide improvements in packaging, response time and cost.
It is therefore an object of the present invention to provide improved power transfer systems for use in four-wheel drive vehicles having a transfer case equipped with a clutch assembly for controlling the torque delivered to the front and rear drivelines in response to changing vehicle conditions.
In accordance with one specific object of the present invention, a full-time power transfer system is disclosed to include a transfer case having a clutch assembly arranged to control speed differentiation and torque biasing across an interaxle differential, sensors for detecting and generating sensor signals indicative of various dynamic and operational characteristics of the vehicle, and a controller for controlling actuation of the clutch assembly in response to the sensor signals. Upon the occurrence of traction loss, the clutch assembly is automatically actuated for limiting interaxle slip while transferring increased drive torque to the non-slipping driveline. Under a preferred adaptive control scheme, the actuated condition of the clutch assembly is controllably modulated between its non-actuated and fully-actuated limits for automatically varying the magnitude of speed differentiation and torque biasing across the interaxle differential in response to changes in the dynamic and operational characteristics of the vehicle.
In accordance with a related object of the present invention, the full-time power transfer system may further include means for establishing a locked four-wheel drive mode in addition to the full-time four-wheel drive mode. To this end, a mode select mechanism is provided for permitting the vehicle operator to select one of the full-time and locked four-wheel drive modes and generate a mode signal indicative thereof. The mode signal is delivered to the controller for use in controlling actuation of the clutch assembly. When the locked four-wheel drive mode is selected, the clutch assembly is fully-actuated for preventing speed differentiation across the interaxle differential and thereby delivering non-differentiated drive torque to the front and rear drivelines. When the full-time four-wheel drive mode is selected, the actuated condition of the clutch assembly is controllably modulated between its non-actuated and fully-actuated limits. Thus, the power transfer system of the present invention offers the vehicle operator the option of selecting the specific four-wheel drive mode best-suited for operating the motor vehicle during normal or adverse road conditions as well as for off-road use.
Pursuant to another specific object of the present invention, an on-demand power transfer system is provided for establishing an on-demand four-wheel drive mode and includes a transfer case having the clutch assembly arranged to automatically transfer drive torque from the primary output shaft to the secondary output shaft. As a related object, the on-demand power transfer system can include a mode select mechanism for permitting selection of one of the on-demand four-wheel drive mode and a part-time four-wheel drive mode. When the part-time four-wheel drive mode is selected, the clutch assembly is fully-actuated for locking the secondary output shaft for common rotation with the primary output shaft.
As a further feature of the present invention, each transfer case can be equipped with a gear reduction unit and a synchronized range shift mechanism that permit xe2x80x9con-the-movexe2x80x9d shifting between high-range and low-range drive modes. Accordingly, the synchronized range shift mechanism permits the vehicle operator to shift the transfer case between the high-range and low-range drive modes without stopping the vehicle.